By Dr Deborah Robertson-Andersson.
Last week we went diving in Long beach Simonstown. We happened to look at a clump of seaweed that was washed up on the beach and had an unusual number of Feather stars crawling around on it. On closer inspection an amazing sight was revealed, baby Feather stars in the process of metamorphosis (change).
In many ways, Feather stars (aka crinoids, owing to their membership in the Class Crinoidea) are among the most mysterious of echinoderms. We know them as fairly stationary flower-like animals. Feather stars evolved more than 300 million years ago in warm, clear shallow seas from animals called sea lilies. They originated from present day England. Sea lilies, or crinoids, are in fact animals. Each sea lily has a long stem attached to the seabed by root-like projections, with a cup-shaped body at the top and long branched arms. Sea lilies which are found mostly in the ocean depths are very similar to the Carboniferous fossils (Palaeozoic era). They evolved from sea lilies and their stem-less cousins. Feather stars are very common around our shores.
There are more than 500 species of Feather stars in the world and only 17 found in South Africa. They are found from the intertidal zone to several thousands of meters in depth. Taxonomically, Feather stars belong to echinoderms (Greek, literally meaning “spiny skin) together with sea urchins, starfish, brittle stars and sea cucumbers. The body of a Feather star displays a five-fold symmetry in their adult form and a skeleton of calcite plates, which are key characteristics of echinoderms.
Feather Stars look a bit like some of the brittle stars, especially those that also lift their arms into the water to feed on suspended material in the water. However, they are easy to tell apart: brittle stars have only five arms, and are not attached by cirri (a slender filament). While Feather stars have 10 to 120 arms, and these numbers differ between species and change during various growth stages; five arms extend from the central disc and each branches once or several times. Each arm has pinnules (a part resembling a barb of a feather) extending from both lateral sides.
Feather stars are suspension feeders, feeding on tiny planktonic organisms e.g. diatoms etc as well as organic particles which are suspended in the water column. They use their arms to gather the food and because of this Feather Stars are common where there are moderate currents to bring food, but not too much silt to clog their feeding tube-feet. When feeding; a food particle will touch one of the tube feet trapping it in mucus. This is then passed down the centre of the arm and across the top of the body to the mouth. Because of the way they feed Feather stars prefer to be above the bottom and its sediment. They often congregate on vertical rock faces, and sometimes even perch on other animals.
The arms are unexpectedly mobile; and have well-developed muscles, ligaments and nerve cords so they not only curl inwards when the animal is resting or are spread out and upwards to feed, but are also used for swimming.
The baby Feather Star life is very interesting, because its first few months are spent as a stalked form similar in structure to a sea lilies. Feather stars mostly have separate sexes, but a few are hermaphroditic. The adults produce eggs in spring and early summer in gonads on the pinnules. Gonads appear as swellings on special pinnules of the arms known as genital pinnules. Genital pinnules usually occur on all arms, but tend to concentrate in the lower third of each arm. Male individuals can be recognised by the creamy white colour of their genital pinnules, and females by pink or orange-coloured pinnules. A single female may spawn over 200,000 eggs.
The larvae will swim, without feeding for 4.5 – 10 days while they chose a site to attach to a rock or pebble, shell, seaweed, or other animal. The larvae swim in an unusual path due to four bands of cilia (hairs) which provide propulsion. The spinning motion helps the larva test the chemical and physical features of its attachment site. At the front end of the larva there is an adhesive pit which contains cells that secrete attachment cement when the larva is ready to settle. Shortly after settling the larvae will develop into a tiny stalked juvenile known as a cystidea. One of the reasons we noticed the Feather stars is that there were so many clustered together. It turns out that the larvae actually search for a chemical cue from the attachment cement of other larvae.
About 16 days from settlement (21 days from hatching), cracks appear in the upper part of the cystidean, and tube feet eventually appear from the cracks (see figure on right). The stalked juvenile now begins to feed and is known as a pentacrinoid a miniature but clearly recognisable relative of the fossil and deep-sea species. About 27 days after settlement the pentacrinoid is about 3 mm in height and can start to feed. The juvenile Feather star can spend up to 6 months in this stage at which time it will be 12 millimetres in height, and will have grown its roughly 250 tube feet for feeding. The crown of tentacles breaks free and the juvenile Feather star crawls down the stalk and adopts the adult mode of life.
Studies’ looking at how much energy is contained in the egg for the development of all the different stages show that 80 % of the eggs energy is used in 25 days before the Feather star is able to feed for the first time. While this reduction in egg energy is fairly normal what is amazing is that a Feather star egg has 90 – 700 times less energy than other echinoderm eggs of comparable size!
Why did the free-swimming Feather stars evolve? The stalked sea lilies held and still hold their body and arms high above the seabed, away from the sediment and in the food-bearing currents, on stems that could be two metres long. But some 100 million years ago, new species evolved that could crush hard shells: crabs, lobsters and fish. It is only a guess that this was what made the shallow seas uninhabitable for the sea lilies, but it makes sense. Although a stalked crinoid does not look like a particularly nutritious meal, it would be easy prey for anything hunting for food. After the new predators became common, stalked crinoids are found only in deep water; in the shallow seas they were replaced by species without stalks, Feather stars which could swim to some extent and so were less vulnerable.